The temperature dependence of the electrical conductivity at low temperatures for a hopping mechanism, where variable range hopping (VRH) prevails, is described by the well-known expression [1]}, [2]}
\(\sigma (T)=\sigma _0\exp \left[ -\left(\frac{T_0}{T}\right)^\alpha \, \right],\) 
\(T_0=\frac{\beta }{g(\mu )\xi ^d}\ ,\) 
